1. Field of the Invention
This invention generally relates to drill motors and particularly relates to self-feeding drill motors used in aircraft manufacturing.
2. Description of the Prior Art
Aircraft manufacturing requires a high level of precision in drilling holes. A guide template having attachment points for a connector at the front of a drill motor is attached to the material to be drilled. A self-feeding drill motor has a feed mechanism for advancing a drill bit into the material while the drill stays attached to the template.
Typically, an air-powered drill motor is used. The typical advancing mechanism, though, is a mechanical system comprising a threaded collar or other positive-feed system for feeding the drill bit into the material at a given rate of spindle advancement. The feed rate is adjustable; however, the force between the bit and the material is usually not monitored or adjusted. As the cutting edges on the drill bit dull, the bit generates a greater amount of thrust as it is fed through the material. If the feed rate is not adjusted to compensate for the drill wear, more force is induced into the material. This force can cause problems in drilling the hole and result in unsatisfactory results, as the drill bit may break through the back side of the hole.
Another feed system uses air cylinders that are parallel to and eccentrically offset from the axis of rotation of the drill motor. The connector for the drill is on the forward end of a feed arm, which is releasably attached to the template. The drill motor is operated to rotate an attached drill bit, and the feeding mechanism is engaged to advance the drill bit into the material. The drill motor and bit move relative to the arm, template, and material.
Due to the offset of the feeding system, a moment can develop around the connection to the template that originates from the thrust between the drill bit and the material. This moment tends to bend the feed arm, causing the drill motor and bit to move out of alignment with the template. This may cause the bit to fail early or produce an unsatisfactory hole.
A method of drilling and drill motor are provided for use with a stationary template attached to a material to be drilled. The drill motor has a frame having a handgrip and within which a movable carriage or motor assembly is located. The motor assembly contains an air-powered motor for rotating a drill bit attached to the motor. At least two air cylinders are located in the motor assembly on opposite sides of the drilling axis and are used to move the motor assembly within the frame. The force that can be applied to move the motor assembly is adjusted by adjusting the airflow to the cylinders. Hydraulic restrictions control the maximum speed at which the motor assembly moves. An auto-return valve and adjustable clamp and rod provide a means for automatically returning the motor assembly to its rearward position once a desired drilling depth has been reached. User controls are provided at the grip for operating the feed system and for manually retracting the motor assembly.